1. Field of the Invention
The present invention relates to a process for the spark erosion machining of a workpiece which has a cylindrical orifice of any cross-section, and more particularly to the spark erosion machining of a die used for the extrusion of metals.
2. Description of the Prior Art
Machining by spark erosion (or sparking) involves producing sparks through a dielectric liquid, for example a hydrocarbon, between a conducting electrode having a shape which is complementary to the removal of material to be obtained on the blank to be machined. It is known that surfaces obtained by spark erosion have a certain roughness which increases on an increase in the speed of operation (or of removal of material) of the process. Greater roughness is thus obtained by faster machining. It is known, furthermore, that the electrode used wears out in proportion with the speed selected, the wear tending to be greater when less roughness is desired. When an impression is machined by spark erosion, it is not possible to obtain immediately a sparked surface which is only slightly rough since the slow speeds required would lead to rates of removal of material which are too slow, thereby resulting in excessively long machining times. Moreover, it is not possible to obtain the corresponding shape directly due not only to the wear of the electrode during operation but also to problems with the circulation of the dielectric liquid (sometimes almost non-existent circulation or, usually, in the opposite direction to the penetration of the electrode for average or fast speeds), all these factors tending to enlarge the impression made. Thus, a cylindrical electrode generally leads to a conical impression. It thus becomes necessary to use several different electrodes each in succession for rough-shaping, semi-finishing and finishing, the last two electrodes gradually reducing the roughness and conicity of the impression owing to the use of decreasing speeds. The under-dimensioning of the successively used electrodes relative to the final impression to be obtained is smaller and smaller so as to take into consideration a phenomenon known in spark erosion as the difference between the lines of pass or of passage, which results from the minimum quantity of material to be removed by a pass made at a given speed to totally eliminate (by replacing it with a slighter one) the roughness obtained in the preceding pass at higher speed, and the working distances between electrode and workpiece for each of the two different speeds considered. It is also possible to use, with the same aim and for the same impression which preferably opens out, a long and tiered electrode having different amounts of under-dimensioning corresponding to the different speeds and pass lines selected for the rough-shaping, semi-finishing and finishing operations. If the electrode is of a complicated shape and the risk of problems in production and control as well as a high cost cannot be accepted, it is necessary to seek other solutions. Thus, it is common practice to adopt a compromise, if possible, and to make one sparking pass which is fast enough to obtain an acceptable speed, while at the same time avoiding the need for geometry and roughness which are too crude. Such a compromise produces inadequate geometry and roughness, necessitating, in the case of extrusion dies, a long and awkward filing operation to finish them off, all the more so when it takes place after the thermal treatments which are carried out after spark erosion to reduce its hardening effect, these thermal treatments obviously causing deformations.
However, various other systems have been developed to provide a better solution to certain problems encountered in the course of spark erosion and, in particular, to make several passes with the same electrode. These are systems which allow a continuous movement of lateral circular translation of radius r to be communicated to the electrode (or to the article which is to receive the impression). The radius r can be adjusted from 0 to a certain value to take into consideration the different lines of passes. Any other controlled and adjustable lateral displacements can be communicated to the electrode, and it is possible for this faculty of adjustment to be used for various purposes (to improve the machining conditions by better washing of the space between the electrode and workpiece and to improve the response to short circuits, for example) and in particular, for various stages in the progress of the spark erosion operation to replace the action of the various successive electrodes. Such systems provide the flexibility which allows practice of the following prior methods.
A first prior art method involves performing the operations of descent (firstly) and of continuous circular translation (secondly) in a manner which is completely separate in time. When the electrode is positioned in the impression which it has previously produced, the descending movement of the spindle of the machine is blocked so that it can spark by using only the continuous movement of circular translation. This movement takes place with a gradual variation of the radius r of the circle of translation which is increased from zero by increments of the order of 0.01 mm and with visual following of the progress of the work according to the state of the parameters of voltage and intensity of the sparking current until a value which has been determined beforehand as a function of the selected speeds and pass lines is reached. This value is reached in one or more stages, depending on the number of speeds used to reach the desired state of finishing. This increase is made by various means (electric or other), but on the basis of an independent manual command which can be pulsed and could only be automated using a sophisticated control device. In fact, it is necessary in any spark erosion operation for the distance between the electrode and workpiece to be permanently controlled with precision, particularly when approaching the finishing speeds. If the distance is too great, sparking does not take place and, if it is too short, a short circuit is produced between the electrode and the workpiece and it is necessary to stop the operation to clean the surfaces. Thus, without a control system, such as would exist if there was only a single descent, the work demands a permanent operator, is very awkward to supervise and only has a very slow speed relative to the average speeds of removal of material normally obtained in conventional descending work as is particularly evident in the speeds for semi-finishing or finishing operations. Another disadvantage is that, in the circular translation phase, the electrode has a fixed level and therefore transmits its own shape, including wear, to the eroded wall.
A second method involves sparking by carrying out both the operation of descent and the operation of continuous circular translation simultaneously, the value of r being constant throughout the entire descent and being adjusted before-hand as a function of the speeds and the selected pass lines, it being possible to repeat the operation several times with values of r which increase depending on the number of speeds and of passages adopted in order to reach the desired state of finishing. This method gives rise to a certain number of difficult problems which can only be solved with the aid of a sophisticated servo-control system which is integrated into the assembly. In fact, as the impression to be obtained is usually of various shapes and is usually not circular, the quantity of metal to be removed is not distributed uniformly in each of the elementary sectors which is covered by the circular translatory movement (of the electrode or of the article to be machined) to such an extent that the descent cannot be performed at the same speed in all the points of the trajectory of this movement. The depth to which the electrode has to operate consequently varies constantly along the trajectory of circular translation and, since the distance between the electrode and workpiece has to be adjusted with precision, the system which controls the descent of this distance, which normally exists on any spark erosion machine, permanently seeks the ideal value but only finds it very rarely. Under these circumstances dead times are significant and the average sparking speed is limited to a few units as a percentage of its normal value due to possible short-circuits or the abnormally high distances between the electrode and workpiece. Furthermore, the erosion of the impression is often incomplete. In fact, the differences of depths of work of the electrode tend to increase in proportion with the progress of the operation and, when the zone which is spark eroded most quickly is completed, the electrode opens into space at this point. The control of the descent of the electrode results in an increase in the speed of descent until the end of the run and this stops the operation. Since the movement of circular translation is relatively slow, the electrode and the sectors of the circle of translation where the work lags behind do not have time to harmonize to prevent this stoppage. To sum up, it can be said, in a simplified manner, that the electrode passes through the first hole which it meets on the circular trajectory and leaves its work incomplete. The above-mentioned disadvantage which is exposed in the case of an opening impression would arise almost under the same conditions for a recessed impression.
The various disadvantages mentioned in this second method, which are still more particularly perceptible in the impressions having parallel lateral faces or in the slightly conical impressions, are avoided in principle by making use as indicated above of an assembly of spark generator, apparatus for the control of circular translation, and a device for controlling the speed of descent of the electrode. The required control system is arranged to communicate with each point of the electrode over a trajectory inscribed on the surface of a cone having a vertical axis which is subjected to the machining conditions, ri, the momentary radius of giration of each point of the electrode not being perfectly constant but being controlled, the value R of the radius of the base of the cone being selected according to the requirements of the machining range (speeds and pass lines) and fixed during the entire descent, the speed of rotation around the axis of the cone being subjected to the quantity of the material to be moved over the trajectory performed.
However, such a sophisticated system cannot easily be adapted to conventional machines and therefore demands the purchase of new machines in which the circular translation device and its connections with the other functions of the assembly are initially designed in an integrated manner. The process according to the invention allows the various disadvantages mentioned above to be overcome by making use of a single electrode for the entire machining operation. It can be carried out with a boring head or circular translation head (to impart the desired movement to the electrode) or with an orbital movement work-holding chuck (to impart the desired movement to the workpiece to be spark eroded), these various materials being commercially available, or with any other system allowing controlled lateral shifting to be communicated to the electrode or to the article to be machined. It can be adapted to conventional spark-erosion machines, subject to dimensional restrictions which can possibly arise in the case of the smallest machines because it only makes use of the control of the distances between the electrode and workpiece in descent which normally exists on all machines. The present process is therefore very easy to automate.